1. Field of the Invention
The present invention relates to making an engineering change to a printed wiring board which mounts electronic components for use in high-performance electronic equipment such as computers. With an ever-increasing integration density of electronic components as in large-scale integrated circuits (hereinafter called LSIs), the demand is also increasing for a printed wiring board which can mount LSIs and connections between LSIs at high density. A printed wiring board, used especially for large-scale computers and super computers, is becoming more and more complicated both in logical design and manufacturing process and eventually becomes subject to an engineering change.
Accordingly, an economic and effective method of making an engineering change to printed wiring boards can remedy poor manufacturing and can easily deal with a change in design.
2. Description of the Related Art
FIGS. 1(a)-1(b) show a method of making an engineering change to a printed wiring board, practicing the first related art of the present invention. FIG. 1(a) is a sectional view of a printed wiring board of the first related art. FIG. 1(b) is a top plan view of a printed wiring board of the first related art.
An etch-cut part 14 and a wire-bonding pad 15 are provided between a pin pad 4 and a through-hole land (or via pad) 13 on a printed wiring board 3. The pin pad 4 is for mounting an LSI pin. The through-hole land (or via pad) 13 leads to an inner layer 5 through a through-hole (or via) 12. To change wiring of the printed wiring board 3, the etch-cut part 14 is cut off to disconnect the wire-bonding pad 15 and pin pad 4 from the inner layer 5, and a discrete wire 8 (FIG. 1(b)) is connected to the wire-bonding pad 15.
FIGS. 2(a)-2(b) show a method of making an engineering change to a printed wiring board, practicing the second related art of the present invention. FIG. 2(a) is a sectional view of a printed wiring board of the second related art. FIG. 2(b) is a top plan view of a printed wiring board of the second related art.
A via pad 16 and a wire-bonding pad 15 are each connected to a pin pad 4 which mounts a pin 2 of an LSI 1. To change wiring of the printed wiring board 3, the etch-cut part 14, connecting the via pad 16 and pin pad 4, is cut by applying a laser beam, for example, to disconnect the pin pad 4 from the inner layer 5. A single wire 8 is placed between the wire-bonding pad 15 and a junction pad 17 and then, a wire 18 terminated in the characteristic impedance (e.g. twin lead is placed between the junction pad 17 and a destination junction pad (not shown).
However, a printed wiring board practicing the former method requires, as shown in FIGS. 1(a)-1(b), a large area for each LSI pin 2 for engineering change purpose. Therefore, the distance between adjacent LSI pins 2 eventually becomes large, which causes a delay in signal transmission and a decrease in processing speed of the electronic equipment.
A printed wiring board practicing the latter method requires only junction pads 17 near an LSI, which decreases the area required for an engineering change. However, since the etch-cut part 14, via pad 16 and wire-bonding pad 15 are provided between adjacent pin pads 4, there is naturally a limitation in decreasing a pin pitch. Moreover, cutting the etch-cut part 14 using a laser beam may a) fail to disconnect the pattern with a clear-cut section, b) carbonize the polyimide provided as an insulator layer between the signal and ground layers, or c) deteriorate the printed wiring board by damaging a pattern on a lower layer.
Therefore, a problem is that a printed wiring board 3 practicing the conventional methods of making an engineering change prevents high-speed signal transmission and, therefore, high-performance data processing because the pin pitch cannot be small enough as mentioned above. Another problem is that a printed wiring board practicing the conventional methods of making an engineering change is expensive because it requires the above-mentioned construction for each LSI pin 2. Still another problem is that the conventional methods of making an engineering change may deteriorate or damage a printed wiring board.